A NASA-funded team of researchers has announced a breakthrough in hypersonic flow technology, allowing operators to control airflow at the speed of light when a deadly ‘shock train’ occurs.
A shock train is a condition that normally precedes engine failure within a scramjet engine. Now, for the first time, researchers based at the University of Virginia School of Engineering and Applied Science have demonstrated the ability to monitor airflow through a supersonic combusting jet engine using an optical sensor instead of a pressure sensor.
This unprecedented level of sensing and control offers engineers of scramjet propulsion engines used in hypersonic prototype aircraft a whole new way to maintain the performance of engines operating above Mach 5, or five times the speed of sound when a shock train is spotted. Aircraft that operate above this threshold are described as being “hypersonic.”
“It seemed logical to us that if an aircraft operates at hypersonic speeds of Mach 5 and higher, it might be preferable to embed sensors that work closer to the speed of light than the speed of sound,” said Professor Christopher Goyne, director of the UVA Aerospace Research Laboratory, where the research took place.
Sensing Shock Train Critical to Hypersonic Flight
In the press release announcing the airflow sensing and control breakthrough, the team notes that NASA’s experimental X-43, or “Hyper-X” aircraft, was able to fly at hypersonic velocities 20 years ago. The final version of that aircraft, the X-43A, reached Mach 10 in 2004. That’s the fastest speed an air-breathing aircraft has reached before or since.
Unfortunately, that program and the Air Force’s successor, the X-51 Waverider program, used older technology to monitor and control the delicate airflow inside a hypersonic engine. Unlike a ramjet engine used by aircraft flying at supersonic speeds, a hypersonic-capable scramjet engine needs to keep the air flowing through it at supersonic speeds. As a result, monitoring that airflow using a 2004 pressure sensor that operates at the speed of sound by monitoring pressure waves left engineers very little time to react to adverse flow conditions.
“If you are sensing at the speed of sound, yet the engine processes are moving faster than the speed of sound, you don’t have very much response time,” Goyne explained.
The UVA team looked to optical sensors to offer engineers much faster data so they could avoid the dreaded “unstart” condition, in which the airflow at the scramjet’s inlet drops below the supersonic threshold.
“If something happens within the hypersonic engine, and subsonic conditions are suddenly created, it’s an unstart,” Goyne explained. “Thrust will suddenly decrease, and it may be difficult at that point to restart the inlet.”
Optical Sensors Find a Whole New Way to Detect and Prevent Unstart
According to the team’s research published in the journal Aerospace Science and Technology, the team believed that monitoring the flow of air at the inlet using optical sensors might offer engineers an opportunity to modify engine conditions before an unstart actually occurs. However, unlike pressure sensors, these sensors would have to look for an optical signal that an unstart was about to occur, as opposed to the traditional tell-tale pressure wave that pressure sensors can detect.
To test their idea, the team employed one of UVA’s several wind tunnels. Dubbed the “UVA Supersonic Combustion Facility,” this particular wind tunnel can simulate engine conditions for a hypersonic vehicle traveling at Mach 5. Also, unlike tests aboard an actual aircraft traveling at hypersonic speeds, the wind tunnels can operate for extremely long periods of time. This flexibility allowed the researchers to gather an unprecedented amount of data on the performance of their optical sensors.
“We can run test conditions for hours, allowing us to experiment with new flow sensors and control approaches on a realistic engine geometry,” explained Chloe Dedic, an associate professor at UVA and co-author of the published research.
After several hours of trial runs, the team found what they had been hoping for. By sensing the light emitted by the reacting gasses within the scramjet combustor, as well as the flame’s location and spectral content, the team could indeed spot the conditions leading up to unstart conditions. Called a “Shock Train,” this wave formation gave off a spectral signal across all three parameters that the team’s optical sensors could detect.
In effect, by analyzing light spectra, they had found a way to monitor and control the conditions within the scramjet to detect and counteract a shock train before the unstart occurs. Also, because this happens at the seed of light, their optical sensor approach to monitoring this category of airflow could help engineers get ahead of one of the most frustrating problems in air-powered hypersonic flight.
“The light emitted by the flame within the engine is due to relaxation of molecular species that are excited during combustion processes,” explained Elkowitz, one of the doctoral students. “Different species emit light at different energies, or colors, offering new information about the engine’s state that is not captured by pressure sensors.”
Research Advances Efforts to Build a Single-Stage-to-Orbit Craft
Moving forward, the UVA researchers are working on new test configurations with the goal of producing a working prototype optical shock train detector for actual in-flight testing.
“We were very excited to demonstrate the role optical sensors may play in the control of future hypersonic vehicles,” said doctoral student and study first author Max Y. Chern. “We are continuing to test sensor configurations as we work toward a prototype that optimizes package volume and weight for flight environments.”
For the longer view, the researchers note that the ability to achieve hypersonic flight moves the world one step closer to the ultimate goal of a single-stage orbit craft that can take off like a traditional airplane and also fly into space.
“One of our national aerospace priorities since the 1960s has been to build single-stage-to-orbit aircraft that fly into space from horizontal takeoff like a traditional aircraft and land on the ground like a traditional aircraft,” Goyne said. “Currently, the most state-of-the-art craft is the SpaceX Starship. It has two stages, with vertical launch and landing. But to optimize safety, convenience, and reusability, the aerospace community would like to build something more like a 737.”
When pondering whether such a goal can be achieved in his lifetime, Goyne expressed optimism, especially given the huge leap forward he believes his team’s work represents.
“I think it’s possible, yeah,” Goyne said. “While the commercial space industry has been able to lower costs through some reusability, they haven’t yet captured the aircraft-like operations. Our findings could potentially build on the storied history of Hyper-X and make its space access safer than current rocket-based technology.”
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.